Hitherto, the transmission capacity of an overhead transmission line (an overhead power electric transmission line) has been increased by means of increasing the size (diameter) of an electric cable. However, as the cable size increases, the mass of the cable increases, and the required sag of the electric cable increases. That is, sufficient space under the transmission line cannot be provided. Also, when the electric cable size is increased, the wind load of the electric cable increases and exceeds the design load of a transmission line tower. Currently, in a capacity-increasing zone, additional tower segments are added so as to bring transmission line towers to a higher level, in order to contend with the increase in sag.
Meanwhile, one conventional electric cable which exhibits an increased transmission capacity is a gap electric cable in which compressed aluminum wires (trapezoidal aluminum wires) surrounding a steel wire are twisted together, to thereby provide a gap between the steel wire and the aluminum wires. In this structure, tension is received only by the galvanized steel wire, and the aluminum wires receive no tension. This electric cable, which exhibits a linear expansion coefficient at high temperature smaller than that of conventional ACSR (aluminum cable steel reinforced), can attain low sag and an increased capacity; about 1.6 times that of ACSR.
Other conventionally employed electric cables which exhibit an increased transmission capacity include Invar electric cables such as galvanized Inver-reinforced extra-heat-resistant aluminum alloy twisted wire (ZTACIR) employing an Invar wire having a small linear expansion coefficient at high temperature instead of a steel wire, and aluminum-coated Inver-reinforced extra-heat-resistant aluminum alloy twisted wire (XTACIR). Since the linear expansion coefficient of Invar wire is as small as ½ to ⅓ that of a galvanized steel wire generally employed in ACSR, the electric cable produced therefrom exhibits small expansion even at high temperature, whereby a sag equivalent to that of ACSR can be attained. In addition, since the outer diameter of the electric cable is equivalent to that of a conventional electric cable, the wind load of a transmission line tower does not increase.
However, the pile-up work of transmission line tower must be carried out while overhead transmission lines are in an active transmission state, and the work requires a long period of working time as compared with general transmission line tower construction and a very high construction cost.
The gap electric cable, provided with a gap between the steel wire and the aluminum layer, is hanged on the overhead transmission line tower by a different electric cable fastening method. When an electric cable is gripped at the surface thereof, similar to the case of conventional ACSR, only the aluminum layer is gripped, and the gripping force does not transfer to the center steel wire portion. Thus, a special gripping metal fitting or tool is needed, which prolongs a work period and requires professional technicians.
An Invar electric cable is an expensive material (i.e., cost is quadruple that of conventional electric cable).
In countries outside Japan, ACAR (aluminum conductor alloy reinforced), which is a product formed by twisting aluminum wires and high-strength aluminum wires together, is employed. Since the product employs no steel wire, the electric cable obtained therefrom is a lightweight cable, requiring a small sag. However, if a fire (e.g., a forest fire or housing fire) occurs under a transmission cable, an aluminum wire is heated at a temperature higher than the melting point, resulting in breakage of the electric cable due to absence of steel wire.
Meanwhile, carbon nanotube is a substance which is formed of a single-layer graphene (carbon) sheet or multi-layer graphene sheets, the sheet(s) forming a co-axial tubular structure. Carbon nanotube is a material having meritorious properties: ultrafine pore size, light weight, high strength, high flexibility, high current density, high thermal conductivity, and high electrical conductivity. Hitherto, a material wire has been formed from a composite material containing carbon nanotubes and aluminum, and an electric cable has been tried to form from element wires including the material wires.
For example, there has been disclosed a high-thermal-conductivity composite material comprising a discharge plasma sintered product mainly formed of metal powder serving as a base material and, uniformly dispersed in the base material to attain a homogeneous state, a fibrous carbon material formed of a ultrathin tubular material comprising single-layer and multi-layer graphene (see Patent Document 1).
There has been also disclosed an element wire in which a plurality of carbon nanotubes are embedded in the metal base forming the element wire such that the carbon nanotubes are aligned in a controlled orientation (see Patent Document 2).